In the pre-shock gas, we find similar molecular abundances of HCN and HNC, close to \(2\times 10^{-9}\) , and a a relative abundance ratio
HCN/HNC= H\(^{13}\) CN/HN\(^{13}\) C= \(1.1\pm 0.4\) .
These values were derived from a direct to the L1157-mm line profiles with CASSIS, taking into account the line optical depths.
Similar results were obtained when considering the rare isotopologues ratios DCN/DNC (= \(1.1\pm 0.4\) ) and HC\(^{15}\) N/H\(^{15}\) NC
(= \(0.9\pm 0.5\) ). The consistency between these values determined independently, makes us confident about the reliability of our method.
The molecular abundances of HCN and HNC are similar to those reported in previous surveys of dark cloud cores and low-mass star forming cores [1]}, [2]}, [3]}, suggesting that the envelope of L1157-mm has kept memory of the prestellar phase. [1]} pointed out that since no difference is observed between the values measured in prestellar and protostellar cores, the evaporation of HCN and HNC from dust grains does not contribute significantly to the observed emission in the cold gas.
